Swift transformed the Calvin Harris and Rihanna club jam to a stripped-down piano ballad much to the approval of her fans in attendance, many of whom sang along. It was the first time Swift has played the song live. She co-wrote the track earlier this year with then-boyfriend Harris.

Swift used the pen name Nils Sjöberg at the time, but in July word got out that Swift was the actual co-writer. “And she sings on a little bit of it too,” Harris tweeted. “Amazing lyric writer and she smashed it as usual.”

“I wrote the music, produced the song, arranged it and cut the vocals though,” he elaborated. “And initially she wanted it kept secret, hence the pseudonym.”